Recombinase Polymerase Amplification (RPA) is a process in which recombinase-mediated targeting of oligonucleotides to DNA targets is coupled to DNA synthesis by a polymerase (Armes and Stemple, U.S. application Ser. No. 10/371,641). RPA depends upon components of the cellular DNA replication and repair machinery. The notion of employing some of this machinery for in vitro DNA amplification has existed for some time (Zarling et al. U.S. Pat. No. 5,223,414), however the concept has not transformed to a working technology until recently as, despite a long history of research in the area of recombinase function involving principally the E. coli RecA protein, in vitro conditions permitting sensitive amplification of DNA have only recently been determined (Piepenburg et al. U.S. patent application Ser. No. 10/931,916, also Piepenburg et al., PlosBiology 2006). Development of a ‘dynamic’ recombination environment having adequate rates of both recombinase loading and unloading that maintains high levels of recombination activity for over an hour in the presence of polymerase activity proved technically challenging and needed specific crowding agents, notably PEG molecules of high molecular weight (Carbowax 20M molecular weight 15-20,000, and others described herein, particularly PEG molecular weight 35,000), in combination with the use of recombinase-loading factors, specific strand-displacing polymerases and a robust energy regeneration system.
The RPA technology depended critically on the empirical finding that high molecular weight polyethylene glycol species (particularly >10,000 Daltons or more) very profoundly influenced the reaction behaviour. It has previously been discovered that polyethylene glycol species ranging in size from at least molecular weight 12,000 to 100,000 stimulate RPA reactions strongly. While it is unclear how crowding agents influence processes within an amplification reaction, a large variety of biochemical consequences are attributed to crowding agents and are probably key to their influence on RPA reactions.
Crowding agents have been reported to enhance the interaction of polymerase enzymes with DNA (Zimmerman and Harrison, 1987), to improve the activity of polymerases (Chan E. W. et al., 1980), to influence the kinetics of RecA binding to DNA in the presence of SSB (Layery and Kowalczykowski, 1992). Crowding agents are reported to have marked influence on systems in which co-operative binding of monomers is known to occur such as during rod and filament formation (Rivas et al., 2003) by increasing association constants by potentially several orders of magnitude (see Minton, 2001). In the RPA system multiple components rely on co-operative binding to nucleic acids, including the formation of SSB filaments, recombinase filaments, and possibly the condensation of loading agents such as UvsY. Crowding agents are also well known to enhance the hybridization of nucleic acids (Amasino, 1986), and this is a process that is also necessary within RPA reactions. Finally, and not least, PEG is known to drive the condensation of DNA molecules in which they change from elongated structures to compact globular or toroidal forms, thus mimicking structures more common in many in vivo contexts (see Lerman, 1971; also see Vasilevskaya. et. al., 1995; also see Zinchenko and Anatoly, 2005) and also to affect the supercoiling free energy of DNA (Naimushin et al., 2001).
Without intending to be bound by theory, it is likely that crowding agents influence the kinetics of multiple protein-protein, protein-nucleic acid, and nucleic acid-nucleic acid interactions within the reaction. The dependence on large molecular weight crowding agents for the most substantial reaction improvement (probably greater than about 10,000 Daltons in size) may reflect a need to restrict the crowding effect to reaction components over a certain size (for example oligonucleotides, oligonucleotide:protein filaments, duplex products, protein components) while permitting efficient diffusion of others (say nucleotides, smaller peptides such as UvsY). Further, it may also be that the high molecular weight preference might reflect findings elsewhere that as PEG molecular weight increases the concentration of metal ions required to promote DNA condensation decreases. In any case it is an empirical finding that RPA is made effective by the use of high molecular weight polyethylene glycols.
In addition to a need for specific type of ‘crowded’ reaction conditions as described above (reaction in the presence of crowding agents), effective RPA reaction kinetics depend on a high degree of ‘dynamic’ activity within the reaction with respect to recombinase-DNA interactions. In other words, the available data which includes (i) reaction inhibition by ATP-γ-S, or removal of the acidic C terminus of RecA or UvsX, and (ii) inhibition by excessive ATP (Piepenburg et al., 2006) suggest that not only is it important that recombinase filaments can be formed rapidly, but also important that they can disassemble quickly. This data is consistent with predictions made in earlier U.S. patent application Ser. No. 10/371,641. Rapid filament formation ensures that at any given moment there will be a high steady state level of functional recombinase-DNA filaments, while rapid disassembly ensures that completed strand exchange complexes can be accessed by polymerases.
Other processes must be adequately supported in the reaction environment in addition to highly dynamic recombinase loading/unloading. For the benefit of later discussions there now follows a more complete list of factors to note when considering how RPA reaction may be affected by changes in activity/properties of the components:
1. As stated above there must be a high overall level of active, correctly loaded, recombinase-DNA filaments at any given moment to ensure rapid kinetics of invasion and strand exchange. This is required to drive rapid reaction kinetics at low target numbers early in the reaction, as predicted by standard bi-molecular reaction kinetics, as well as to ensure non-limiting quantities of active filaments late in the reaction when targets become highly abundant and could easily out-titrate the loaded filaments.
2. Filaments must be dynamic, capable of rapid disassembly as well as assembly, to ensure that strand exchange processes work rapidly, and to avoid filament ‘lock-up’ in unproductive protein-DNA conformations (should they arise).
3. Recombinases should have a strong preference for single-stranded DNA, and a relatively weaker preference for double-stranded DNA. This ensures the correct partitioning of recombinase onto the oligonucleotides, and is very important in the late phase of the reaction when significant quantities of duplex DNA accumulate. This duplex DNA may otherwise compete too effectively for recombinase and slow the reaction too rapidly. A difference in disassembly rates on duplex DNA would also enhance factor (ii) insofar as accelerating disassembly of productive exchange complexes. Observations consistent with ‘out-titration’ activity of excess duplex DNA, such as decreases in reaction rate late in the reaction, or if excess DNA is present early in the reaction, have been made.
4. Hybridization of single-stranded DNA's to one another must be supported under any given reaction condition. RPA has the potential to generate single-stranded DNA products which may only be converted to new duplex targets following hybridization of the complementary priming oligonucleotide to initiate DNA synthesis. As saturating quantities of single-stranded DNA binding proteins (i.e. loading proteins, single-stranded DNA binding proteins and recombinases) are present in the reaction environment, these hybridization processes must be supported/aided by these proteins. SSB's and recombinases have some melting/hybridization activities on duplex/single-stranded DNA's, and probably demonstrate differential levels of melting/hybridization activity. Thus the relative proportions of recombinase and SSB of loading may influence the rate behaviour for hybridization, and this may also depend on the species of SSB and recombinase employed. If either the SSB or recombinase does not, or only poorly, supports hybridization of single-stranded DNAs to one another, then the reaction may be compromised.
5. The temporal change in reaction composition with regard to pH, anion accumulation, generation of ADP, of AMP, pyrophosphate, and other nucleotide species may be strongly influenced by the recombinase employed. Furthermore recombinases may respond differentially to the ionic and pH environment. Rates of nucleotide hydrolysis affect the accumulation of the afore-mentioned species, and their accumulation may in turn influence the activity in the reaction of recombinases and polymerases. For example accumulation of phosphate and pyrophosphate may inhibit recombinase processes, while the accumulation of ADP (and possibly AMP) can affect DNA on-off kinetics of the recombinase. Notably bacteriophage T4 UvsX protein has been reported to hydrolyse ATP to both ADP and AMP, a property not attributed to other recombinases to date. Recombinases may also hydrolyse dATP, UTP and potentially other nucleotides. Different nucleotides may affect the DNA binding stabilities of complexes on ssDNA and dsDNA, for example dATP has been noted to increase the stability of RecA on ssDNA. Without intending to be bound by theory, the particular properties of a recombinase with respect to its DNA binding domains and nucleotide binding/catalysis domains may have significant impact on reaction rate and effectiveness in generating strong signals late in the reaction.
Previously Established RPA Conditions.
Effective RPA reactions have previously been demonstrated using both E. coli RecA (in a heterologous system with compromised gp32 protein) and with the T4 phage UvsX protein (when combined with the T4 phage UvsY protein) (Piepenburg et al., 2006). In both cases the employment of polyethylene glycol was found to be absolutely necessary for amplification to occur with any useful efficiency when templates were present at concentrations below roughly nanomolar levels (or roughly below the order of about 1010 target molecules per microliter).
Experimentation showed the importance of PEG in stimulating secondary, tertiary and yet further invasion events when using oligonucleotides directed towards the ends of linear templates, said oligonucleotide initially having a 5′ overhang relative to the initial target, but being flush to later targets due to the activity of ‘backfire’ synthesis (Piepenburg et al. U.S. Ser. No. 10/931,916). Fully embedded targets proved to be even more intractable, almost certainly due to the topological constraints associated with the recombination products caused by the outgoing strand being wound unfavourably around the newly formed duplex. Without intending to be bound by any theory, the huge increase in efficiency of initiating replication from these more unstable intermediates in the presence of PEG may depend on stability conferred by the crowding agent on the complexes, on altered DNA conformation and coiling (such as DNA condensation), on much higher association constants for the polymerase gaining access to the intermediates, and/or a very great increase in the frequency of recombination events leading to more ‘chances’ of the polymerase grabbing the intermediate and elongating.
An RPA system utilizing bacteriophage T4 UvsX, T4 UvsY, and T4gp32, a B. subtilis PolI large fragment, and PEG compound (carbowax 20M) is effective for amplifying duplex DNA sequences up to about 1 kilobase in length (Piepenburg et al., 2006). Average doubling times of as little as 40 seconds or less have been attained for fragments of roughly 300 nucleotides, and DNA accumulates to levels useful for detection by a variety of means, even when targets are initially present at levels below 10 copies. Despite this robust behaviour there exists a need for the identification of other recombinases, their associated loading components and single stranded DNA binding proteins, due to the strict necessity for very rapid kinetics and strong signals for the implementation of the RPA system in commercially useful products. The present invention meets these needs and other needs.